Always wear eye protection during science experiments. Never handle sharp objects, open flames, chemicals, or other dangerous objects without adult permission and supervision.
First, and most importantly, go outside! This experiment is very messy.
Place the bottle of Diet Coke on a hard, flat surface, and open the cap. (Note: the next step will be a lot easier if you have a funnel or tube of some sort, but this is optional)
Slide the Mentos into the bottle of Diet Coke all at once, and run away!
Watch your geyser go bananas!